This invention relates to a cobalt-based alloy containing a substantial proportion of chromium and molybdenum together with other alloying ingredients, particularly ingredients which provide a fine oxide dispersion, said alloy being produced by a gas atomization process. The invention is also concerned with a process for producing said alloy and to prostheses formed therefrom.
The alloy of the present invention falls within a class known in the art as "superalloys".
The term "superalloy" is a term of art which generally signifies an alloy having particularly high strength, good mechanical and corrosion-resistant characteristics and a stable microstructure. Of particular interest are those alloys which additionally retain high strength properties (and stable microstructures) following thermal treatments at extremely high temperatures.
The known Vitallium.RTM. alloy is a high corrosion-resistant cobalt/chromium alloy which is used successfully in numerous orthopaedic applications. A typical composition for Vitallium.RTM. alloy is the following:
______________________________________ Element % by weight ______________________________________ Carbon 0.25 Silicon 0.75 Manganese 0.70 Chromium 28.00 Molybdenum 5.50 Cobalt 64.80 ______________________________________
Because of its many favorable properties, for example, high ambient temperature strength and fatigue strength, resistance to wear, bio-compatibility and particularly corrosion resistance, Vitallium.RTM. alloy is used extensively in orthopaedic applications, especially for prostheses. A particularly useful development in the area of orthopaedic implants is the provision of a porous coating in the form of multiple layers of spherical Vitallium.RTM. alloy particles on the surface of a Vitallium.RTM. alloy for the enhancement of implant fixation. However, with the advent of porous coating, some of the fatigue strength of cast Vitallium.RTM. alloy may be lost due to the elevated temperature required for sintering. Accordingly, there is a need to provide a Vitallium.RTM. alloy for hip implants wherein the fatigue strength is maximized.
It is known that the properties of a given metal alloy are dependent upon its composition and also upon the manner in which the various alloying ingredients are formed into the final alloy. One method of alloy formation is known as "mechanical alloying" and this method ideally produces homogeneous composite particles with a uniformly dispersed oxide. The process is described in an article entitled "Dispersion Strengthened Superalloys by Mechanical Alloying" by John S. Benjamin, Metallurgical Transactions, Vol. 1 October 1970, p. 2943.
U.S. Pat. No. 3,591,362, issued July 6, 1971 to John S. Benjamin discloses a composite alloy powder formed by the technique of mechanical alloying.
The inclusion of certain selected oxides in the alloy composition can improve the properties of the final alloy and oxide dispersion strengthened (O.D.S.) superalloys made by the mechanical alloying process exhibit high-temperature strength and stability as a result of the presence of stable oxide dispersions which resist thermal damage and permit much greater freedom in alloy design.
U.S. patent application Ser. No. 703,352, filed Feb. 20, 1985 discloses an improved cobalt-chromium superalloy made in accordance with O.D.S. mechanical alloying procedures which has not only the high corrosion-resistant properties typical of Vitallium.RTM. alloy but also excellent room temperature strength (tensile and fatigue) properties which are substantially retained after exposure to severe thermal conditions.
While the improved alloy of application Ser. No. 703,352 has excellent strength properties and high temperature stability which makes it vastly superior to any prior art alloy, said improved alloy has insufficient ductility for conventional hot working.
Surprisingly, it has now been found that an alloy having greatly enhanced ductility and consequential good hot workability, may be obtained when the alloy, having small amounts of oxides and nitrides, is produced by gas atomization and suitable thermomechanical processing rather than the mechanical alloying procedure described above.
Gas atomization of metals is a known technique for producing alloy powders having certain powder characteristics such as average particle size, particle-size distribution and particle shape. These characteristics affect the mechanical properties of the solid alloy which is formed by consolidating the powder. Typical methods of gas atomization are described in the literature; for example, ASM Handbook 9th edition, Vol. 7 Powder Metallurgy p. 25, 38 American Society for Metals, Metals Park Ohio, 1984.